How String Instruments Work

Sound waves are produced by transferring vibrations from one particle to the next. The sound waves we hear usually travel to our ears through the air, but they can also move through liquids and solids. There are many different ways to produce vibrations, and the sound will change based on the source of the vibration and the methods used to produce it.

Chordophones are musical instruments that produce sound by causing strings to vibrate and perhaps amplifying that vibration. The frequency of the vibration determines the pitch of the tone observers hear. Frequency is affected by the length, mass, and tension of the vibrating string.

Chordophones are musical instruments that produce sound by causing strings to vibrate and perhaps amplifying that vibration. Frequency is affected by the length, mass, and tension of the vibrating string. Amplitude is affected by the speed and weight of the force applied to the string. A stronger pluck or faster, heavier bow strokes produce a louder sound.

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Mersenne's First Law: The fundamental frequency of a stretched string is inversely proportional to the length of the string, keeping the tension and the mass per unit length of the string constant.

Length

On some instruments, the length of the string is fixed. Other instruments have a fingerboard or a fretboard which is used as a surface onto which the musician presses the strings to shorten the portion of the string that is free to vibrate. The pitch increases as the string gets shorter, or as the musician moves up the neck of the instrument.

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Tension

String instruments have to be "tuned." How this is accomplished varies a bit from one instrument to the next, but the basic principle is that the string is attached to something that can be turned, such as a tuning peg, and this allows the musician to adjust the tension until the string produces the desired frequency.

Mersenne's Second Law: The fundamental frequency of a stretched string is directly proportional to the square root of the tension in the string, keeping the length and the mass per unit length of the string constant.

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Mass

Larger, heavier strings have a lower pitch than smaller, lighter strings. This also makes the strings of lower-pitched instruments thicker, and the instruments themselves are usually larger.

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Mersenne's Third Law: The fundamental frequency of a stretched string is inversely proportional to the mass per unit length of the string, keeping the length and the tension in the string constant.

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Overtones and Harmonics

When strings vibrate, they produce other sound waves and other frequencies besides the frequency our ears hear. Together, these sound waves are called a compound tone. The frequency that is heard is the fundamental frequency, which is the lowest frequency a string produces, given its length, weight, and tension. The other frequencies are multiples of the fundamental frequency. If you place a finger at one of several specific points on the string, that point will become a node, and other nodes may also form, allowing you to hear a different frequency, or a harmonic. The frequency of each harmonic can be calculated by multiplying the fundamental frequency by the number of nodes produced.

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Sound Box and Sound Holes

Many string instruments have the strings suspended over a hollow body, generally referred to as a sound box and usually made of wood. The instrument is designed in such a way that the vibration of the strings is transferred into the body of the instrument. This causes the air inside the instrument as well as the walls of the sound box itself and any structures inside the sound box (such as a sound post and bass bar) to vibrate as well. The sound waves are then projected out through the sound hole, or holes as the case might be, having been modified in tone and volume by the sound box. The size, shape, and composition of the sound box can alter the sound of the instrument, as can the shape and location of the sound holes. Luthiers (string instrument makers) have experimented throughout the histories of these instruments - centuries in most cases - to determine what design will produce the most desirable sound for that particular instrument. Sometimes, this gradual experimentation results in something so different from its predecessor that it needs a different name. For this reason, we don't really have an official "inventor" for most musical instruments. The process is usually more gradual, more evolutionary than revolutionary, so that no single person can rightfully be given credit.

Plucking or Striking Vs. Bowing

Although the basic principles of how a string instrument functions are the same, the manner in which they are played varies. Some, like the piano or the hammer dulcimer, are played by striking the strings or pressing a key that causes a hammer to strike the keys. String instruments that are struck are therefore often classified as percussion instruments.

Other instruments, like the guitar, are played by plucking the strings. Then, we have instruments that are played with a bow. The symphonic strings (violin, viola, cello, and bass) are often bowed but can also be plucked with the finger or struck with the bow stick (called col legno), bow hair (chop) or with a thin wooden object called a fiddlestick (see video below). When a string is plucked or struck, the force is only briefly applied to produce the intended tone, and the sound quickly fades away. However, the bow continues to move the string for the duration of the intended tone, so the sound is more sustained and fades only after the bow stops moving, or immediately if the bow is stopped in such a way that it dampens the string.